The function of the cardiovascular system is vital for survival. Through blood circulation, body tissues obtain necessary nutrients and oxygen, and discard waste substances. In the absence of circulation, cells begin to undergo irreversible changes that lead to death. The muscular contractions of the heart are the driving force behind circulation.
In cardiac muscle, the muscle fibers are interconnected in branching networks that spread in all directions through the heart. When any portion of this net is stimulated, a depolarization wave passes to all of its parts and the entire structure contracts as a unit. Before a muscle fiber can be stimulated to contract, its membrane must be polarized. A muscle fiber generally remains polarized until it is stimulated by some change in its environment. A membrane can be stimulated electrically, chemically, mechanically or by temperature change. The minimal stimulation strength needed to elicit a contraction is known as the threshold stimulus. The maximum stimulation amplitude that may be administered without eliciting a contraction is the maximum subthreshold amplitude.
Where the membrane is stimulated electrically, the impulse amplitude required to elicit a response is dependent upon a number of factors. First, is the duration of current flow. Since the total charge transferred is equal to the current amplitude times the pulse duration, increased stimulus duration is associated with a decrease in threshold current amplitude. Second, the percentage of applied current that actually traverses the membrane varies inversely with electrode size. Third, the percentage of applied current that actually traverses the membrane varies directly with the proximity of the electrode to the tissue. Fourth, the impulse amplitude required to elicit a response is dependent upon the timing of stimulation within the excitability cycle.
Throughout much of the heart are clumps and strands of specialized cardiac muscle tissue. This tissue comprises the cardiac conduction system and serves to initiate and distribute depolarization waves throughout the myocardium. Any interference or block in cardiac impulse conduction may cause an arrhythmia or marked change in the rate or rhythm of the heart.
Sometimes a patient suffering from a conduction disorder can be helped by an artificial pacemaker. Such a device contains a small battery powered electrical stimulator. When the artificial pacemaker is installed, electrodes are generally threaded through veins into the right ventricle, or into the right atrium and right ventricle, and the stimulator is planted beneath the skin in the shoulder or abdomen. The leads are planted in intimate contact with the cardiac tissue. The pacemaker then transmits rhythmic electrical impulses to the heart, and the myocardium responds by contracting rhythmically. Implantable medical devices for the pacing of the heart are well known in the art and have been used in humans since approximately the mid 1960s.
Either cathodal or anodal current may be used to stimulate the myocardium. However anodal current is thought not to be useful clinically. Cathodal current comprises electrical pulses of negative polarity. This type of current depolarizes the cell membrane by discharging the membrane capacitor, and directly reduces the membrane potential toward threshold level. Cathodal current, by directly reducing the resting membrane potential toward threshold has a one-half to one-third lower threshold current in late diastole than does anodal current. Anodal current comprises electrical pulses of positive polarity. The effect of anodal current is to hyperpolarize the resting membrane. On sudden termination of the anodal pulse, the membrane potential returns towards resting level, overshoots to threshold, and a propagated response occurs. The use of anodal current to stimulate the myocardium is generally discouraged due to the higher stimulation threshold, which leads to use of a higher current, resulting in a drain on the battery of an implanted device and impaired longevity. Additionally, the use of anodal current for cardiac stimulation is discouraged due to the suspicion that the anodal contribution to depolarization can, particularly at higher voltages, contribute to arrhythmogenesis.
Virtually all artificial pacemaking is done using stimulating pulses of negative polarity, or in the case of bipolar systems, the cathode is closer to the myocardium than is the anode. Where the use of anodal current is disclosed, it is generally as a charge of minute magnitude used to dissipate residual charge on the electrode. This does not affect or condition the myocardium itself. Such a use is disclosed in U.S. Pat. No. 4,543,956 to Herscovici.
In using a triphasic waveform, the first and third phases have nothing to do with the myocardium per se, but are only envisioned to affect the electrode surface itself. Thus, the charge applied in these phases is of very low amplitude. Disclosures relevant to this practice are found in U.S. Pat. Nos. 4,903,700 and 4,821,724 to Whigham et al., and U.S. Pat. No. 4,343,312 to Cals et al.
Lastly, biphasic stimulation has been used to produce voltage doubling without the need for a large capacitor in the output circuit. The phases of the biphasic stimulation disclosed are of equal magnitude and duration. Refer to U.S. Pat. No. 4,402,322 to Duggan for details.
What is needed is an improved means for stimulating muscle tissue, wherein the contraction elicited is enhanced and the damage to the tissue adjacent to the electrode is diminished.
Enhanced myocardial function is obtained through the biphasic pacing of the present invention. The combination of cathodal with anodal pulses, of either a stimulating or conditioning nature, preserves the improved conduction and contractility of anodal pacing while eliminating the drawback of increased stimulation threshold. The result is a depolarization wave of increased propagation speed. This increased propagation speed results in superior cardiac contraction leading to an improvement in blood flow. Improved stimulation at a lower voltage level also results in reduction in power consumption and increased life for pacemaker batteries. Lastly, the improved stimulation achieved through the practice of the present invention allows for cardiac stimulation without the necessity of placing electrical leads in intimate contact with cardiac tissue. Standard stimuli delivered to the cardiac blood pool are ineffective in capturing the myocardium because it does not meet the stimulation threshold. While voltage of the pulse generator can be increased, when it does capture it is often so high that it also stimulates skeletal muscles thereby causing a painful twitching of the chest wall when only the heart stimulation was desired. As will be further discussed, through the practice of the present invention, one can enhance myocardial function through cardiac blood pool stimulation.
As with the cardiac muscle, striated muscle may also be stimulated electrically, chemically, mechanically or by temperature change. Where the muscle fiber is stimulated by a motor neuron, the neuron transmits an impulse that activates all of the muscle fibers within its control, that is, those muscle fibers in its motor unit. Depolarization in one region of the membrane stimulates adjacent regions to depolarize also, and a wave of depolarization travels over the membrane in all directions away from the site of stimulation. Thus, when a motor neuron transmits an impulse, all the muscle fibers in its motor unit are stimulated to contract simultaneously. The minimum strength to elicit a contraction is called the threshold stimulus. Once this level of stimulation has been met, the generally held belief is that increasing the level will not increase the contraction. Additionally, since the muscle fibers within each muscle are organized into motor units, and each motor unit is controlled by a single motor neuron, all of the muscle fibers in a motor unit are stimulated at the same time. However, the whole muscle is controlled by many different motor units that respond to different stimulation thresholds. Thus, when a given stimulus is applied to a muscle, some motor units may respond while others do not.
The combination of cathodal and anodal pulses of the present invention also provides improved contraction of striated muscle where electrical muscular stimulation is indicated due to neural or muscular damage. Where nerve fibers have been damaged due to trauma or disease, muscle fibers in the regions supplied by the damaged nerve fiber tend to undergo atrophy and waste away. A muscle that cannot be exercised may decrease to half of its usual size in a few months. Where there is no stimulation, not only will the muscle fibers decrease in size, but also they will become fragmented and degenerated, and replaced by connective tissue. Through electrical stimulation one may maintain muscle tone, such that upon healing or regeneration of the nerve fiber, viable muscle tissue remains. Enhanced muscle contraction is obtained through the biphasic stimulation of the present invention. The combination of cathodal with anodal pulses of either a stimulating or conditioning nature results in contraction of a greater number of motor units at a lower voltage level, leading to superior muscle response.